Any laser weapon will do that on a critical hit. Plasma weapons turn them into a green goo pile.

My favorite though, would most likely be the Perforator. This changes based on my current situation though. I use the Gauss Rifle, and the Wazer Wifle, or the Metal Blaster as well. If I have enough Alien Power Cells, I might use the Alien Disintegrator, if the situation calls for something quick and dirty.

The Perforator is my main simply because of V.A.T.S. damage. I play a Stealth based character, so using the Perforator with the Chinese Stealth Suit, my first attack does 104 damage per bullet, and because the weapon fires two rounds in V.A.T.S., and both are considered sneak attacks, and because I use the Better Criticals perk, the total damage comes to 232 for the first attack, more than even the Gauss Rifle, and enough take out any enemy, barring Reavers, Behemoths, and Overlords.

As well as this, I like the Terrible Shotgun.

Taken from The Vault - While in perfect condition, and with a Small Guns skill of 100, the weapon deals 80 damage, and an extra 40 damage for critical hits. However, this weapon fires 9 pellets. Because of this, when the weapon is fired outside of V.A.T.S. and scores a critical hit, the critical hit for each pellet is counted individually. This means that (if all pellets make contact), the weapon will deal 440 damage (80 base damage, plus 9 extra 40 critical damages). In Sneak Mode, this damage is doubled, increasing it to 880 points of damage. And if the player has the Better Criticals perk, the damage will increase to 1240. If the player further increases the damage via a Headshot , then that damage doubles again to 2480, enough to one-shot a Super Mutant Behemoth.

However, because it is NOT a silent weapon, and because the above can be extremely hard to pull off, I use it sparingly. Especially because shotgun shells can be pressed into .556, which I use more of.